<style>
  * {
    padding: 0;
    margin: 0;
  }
  .container {
    width: 100vw;
    height: 100vh;
    background-color: #000;

    display: flex;
  }

  .menu {
    width: 200px;
    height: 100%;
    background-color: red;
  }

  .main {
    flex: 1;
    background-color: blue;
  }
</style>

<div class="container">
  <div class="menu"></div>
  <div class="main">内容</div>
</div>
